When a display is exposed to external light such as various illumination and natural light, an image formed inside the display is not clearly focused on an eye, thereby causing deterioration in contrast of the display. Due to such deterioration in contrast, a person has a difficulty in viewing a screen, and feels fatigue in the eye, or suffers from a headache. For this reason, there is a strong demand for anti-reflection.
In a substrate on which a single-layer anti-reflective film is formed, when an index of refraction of the substrate is defined as ns and an index of refraction of the single-layer anti-reflective film is defined as n, a minimum value of reflectance R of the anti-reflective film is represented by (ns−n2)2/(ns+n2)2. Since the reflectance R has the minimum value under the condition of n2=ns, the single-layer anti-reflective film has lower reflectance as the index of refraction n thereof is closer to (ns)1/2. Generally, considering that an index of refraction ns of polyethylene terephthalate used as a substrate in transparent conductive films is about 1.54, it is desirable that an index of refraction n of an anti-reflective film be as close to a range of about 1.22 to about 1.24 as possible in order to reduce a reflectance R of the anti-reflective film.
In typical anti-reflective films, an anti-reflective layer is mainly disposed on a transparent substrate. For example, Japanese Patent Publication No. 2002-200690 discloses a 3-layer structure in which a hard coating layer, a 1 μm or less thick high index of refraction layer and a low index of refraction layer are stacked in order from a transparent substrate.
In addition, to simplify a manufacturing process, Japanese Patent Publication No. 2000-233467 discloses a 2-layer structure in which the hard coating layer and the low index of refraction layer are stacked with omission of the high index of refraction layer from the anti-reflective layer as set forth above.
Due to development of hollow silica particles, which are a low refractive material, low refractive coating materials having an extremely low index of refraction have been studied. However, a low refractive coating material developed using an existing acrylic resin has not reached an index of refraction of 1.22 to 1.24, which is a theoretically optimum value for anti-refraction. To improve this problem, although efforts to reduce an index of refraction by addition of a fluorine-containing polymer material have been made, there occurred a problem of excessively high surface energy of a coating surface. In addition, since compatibility between the hollow silica particles and the acrylic resin is not good, in order to overcome this problem, the silica particles need to be subjected to surface treatment allowing the particles to be bound to the acrylic resin.
Korean Patent Publication No. 2004-0070225 discloses a coating composition including a silica precursor obtained by hydrolysis and polycondensation of an alkoxy silane. However, since a method of condensation and thermal curing of a silane compound does not provide sufficient curing only by low-temperature and short-term thermal curing, high-temperature or long-time curing is required for sufficient curing density of the silane compound. Thus, the method has problems such as increased manufacturing costs, deteriorated productivity, damage to a plastic film substrate, cracking of a hard coating layer due to shrinkage thereof by heating, and the like.
In addition, although a technique for manufacturing a low refractive coating layer by introduction of a fluorine-containing alkoxy silane is also considered, there is a limit in applying the technique due to problems, such as phase separation and the like.
In manufacture of touch sensor panels and the like, it is necessary to impart suitable adhesion to a film surface, and control of adhesion of a surface of a coating layer by adjustment of surface energy of the coating layer may be considered for this purpose.
Therefore, there is a continuous need for development of a method which allows a silica coating layer having a low index of refraction and suitable surface energy to be formed on a substrate without phase separation.